Fingerprint sensing is now widely used for identification and verification purposes. For this, a person's fingerprint is acquired by a fingerprint sensing device whose output is processed and compared with stored characteristical data of one or more fingerprints to determine whether a match exists. Most optical fingerprint sensing apparatuses incorporate a prism and take advantage of a frustrated total internal reflection (FTIR) effect to image the fingerprint, such as is described in U.S. Pat. Nos. 2,195,699 and 5,416,573. In the prior art, the need to block ambient light from a fingerprint scanning device is considered to be an important issue and multiple solutions have been proposed. For example, in U.S. Pat. No. 7,119,889 an ambient light shield comprising a cover that sits over the platen is suggested where said cover blocks ambient light of the wavelengths used by the fingerprint scanner to image a fingerprint. As a second example, contact fingerprint scanner based upon FTIR typically incorporate a spectral filter, typically a series of dielectric films on a glass substrate, such that the filter behaves as a bandpass filter. As such, said filter allows light used by the fingerprint scanner to pass through to the sensor of the scanner and light outside of this spectral band, e.g., ambient light, is blocked.
An alternative to a FTIR fingerprint scanner is a contact fingerprint scanner. As described in U.S. Pat. Nos. 5,991,467 and 7,369,690, optically sensing of a fingerprint may also be performed using a photoelectric sensor 20 such as described in and shown schematically for example in FIG. 1. The prior art describes backlight illumination 21a from a source 21 is transmitted through transparent regions of array 20a to strike a finger 22 that is placed on a platen 23. In regions where the ridges 8 of the fingerprint make contact with platen 38, the light transmits into the finger, scatters (ray 25) and is detected by a two-dimensional (2D) array of light sensitive detectors 24. The valleys 7 of the fingerprint of finger 22 when in contact with fingerprint sensor 20 create an air boundary that reflects illumination (ray 28) and reflected light does not get detected by light sensitive detectors 24. The remaining illumination light impinging of areas of fingerprint valleys transmits to the skin and although some is reflected, a significantly reduced amount of light returns to the light sensitive detectors in comparison with that of the ridges and hence an image of the fingerprint consisting of ridges and valleys is created. The light sensitive detectors 24 each have a capacitor or capacitance which stores the accumulated charge of the detector 24 in accordance with the amount of the reflected light 25 the detector 24 receives. The amount of the light 25 received into each of the light sensitive detectors 24 differs according to its position from which the light is reflected because a reflectance between a light 25 reflected from a ridge 8 portion that is protruded portion of the finger, and a light 28 reflected from a valley 7 portion that is recessed portion of the finger 22 is different from each other, where the ridges and valleys of FIG. 1 have been drawn in an exaggerated scale in order to clarify the operation of the device. Transistors 26 are provided for each of the detectors 24. Each transistor 26 switches to readout out the amount of the electron charge stored in the capacitor of its associated detector 24. These switching transistors 26 may be thin film transistors known as TFTs, and light sensing detectors 24 may be thin-film based PIN photodiodes.
Platen 23 may be provided by the surface of a thin protective layer 27 over a substrate or transparent backplane 29 having detectors 24, and other electronics, including transistors 26, electrical connections, and other elements, typical of TFT-based sensors for enabling their operation. Fabrication of sensor 20 may use amorphous silicon technology formed on a backplane 29 of glass. Backlight illumination 21 passes through substrate 29 and the non-opaque areas (e.g., areas that do not contain detectors 24, transistors 26, electrical connections and other elements) upon substrate 29. Detectors 24 are opaque on the side facing substrate 29 so that illumination light 21a from source 21 cannot be directly detected, but only detected because of a reflection or scattering.
Detectors 24 are referred to hereinafter as light sensing pixel elements (or pixels) 24 of the two dimensional sensor array 20a, since each detector senses light in accordance with one pixel (when readout by other electronics on the chip of sensor 20) of a two-dimensional image representative of a fingerprint of the subject finger 22 or finger(s), palm, thumb, or other skin of a person. Since the finger 22 is in close proximity to the light sensing pixels of array 20a, no imaging optics are used, thus the term of a device using this photoelectric sensor to capture a fingerprint image is referred herein as a non-imaging contact fingerprint sensor 20, where such sensor has a two-dimensional sensor array 20a of light sensing pixels. Fingerprint contact sensors where TFTs provide transistors 26 are referred to herein as TFT-based fingerprint contact sensors. However, heretofore the improvements provided by the present invention, a commercially useful non-imaging contact fingerprint sensor has not been successfully developed for use in fingerprint scanners. Such being desirable since avoiding the need for imaging optics of a FTIR fingerprint scanner would enable the scanner to be more compact and lightweight, especially useful for mobile fingerprint scanners.
For a fingerprint sensor that is based upon a non-imaging contact approach as depicted in FIG. 1, such sensor is significantly more compact and lightweight than FTIR fingerprint sensors, a plus for mobile applications, however, another general requirement for any mobile application is to have as low as possible power consumption so as to maximize life of a battery typically present. One source of drain on such battery in the scanner's internal illumination source, such as shown by illumination 21 in FIG. 1. It thus would be desirable to provide a non-imaging contact sensor which does not require internal illumination for operation, or that use of an internal illumination source is optional, thereby lowering overall power consumption of the scanner.